May - July 2010

Outlook Graphic: GIF   PDF 

Latest Seasonal Assessment - During April, moderate to severe hydrological drought has persisted across the northern Rockies and the upper Midwest. Abnormal dryness has developed in northeast Minnesota, northern Ohio, western New York and into the Tennessee Valley due to below-average streamflow percentiles and soil moisture anomalies. Though precipitation is expected through much of the remainder of April for the northern Rockies, drought persistence is forecast for Idaho and western Montana due to forecasts of poor streamflows and soil moisture values into the May - July season. Some improvement is indicated in southwest Wyoming with improvement indicated for northern Colorado, due to more favorable forecasts of soil moisture combined with slightly more favored prospects for precipitation through the latter part of April into May. Drought persistence is forecast for the Great Basin and northeast Arizona as this region begins to enter the climatological dry season. Across the upper Midwest eastward into the eastern Great Lakes region, forecasts of below-median precipitation indicated in the next two weeks and in the May precipitation outlook combined with forecasts of below-average soil moisture profiles during May - July lead to forecasts of drought persistence in areas already in at least moderate drought, and drought development from northeast Minnesota, central Wisconsin and into the eastern Great Lakes region. Drought development is also indicated across southern Ohio, central Kentucky into northern Tennessee, with drought persistence forecast for the area in central Kentucky already in moderate drought.

Ongoing areas of moderate to exceptional drought have affected the leeward side of the Hawaiian Islands, with drought conditions expected to persist through the May - July season. Across Alaska, below-average snow water equivalent values, warming June and July temperatures combined with slightly enhanced odds for below-median precipitation support drought development in central Alaska.

Tools used in the U.S. Drought Outlook (USDO) included the official CPC precipitation outlook for May 2010 and the long lead forecast for May - July 2010, the four-month Palmer drought termination and amelioration probabilities, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, El Niño precipitation and temperature composites for May - July 2010, climatology, and initial conditions.

Moderate to severe drought continues in northwest Wisconsin, and moderate drought has also expanded into much of northern Michigan, while abnormal dryness has spread northwestward into the Arrowhead region of Minnesota. These conditions are largely due to very low streamflows, below-normal soil moisture profiles and lack of measurable precipitation in this region. Though some precipitation is expected in the short-term, CPC's 6-10 day, 8-14 day and monthly precipitation forecasts indicate greater odds for below-normal precipitation in this region. These precipitation forecasts combined with warming temperatures and below-average seasonal soil moisture and runoff forecasts suggest drought persistence is most likely in this region. The same forecast reasoning also applies to a region of drought development now designated for the remainder of northern Michigan not currently in moderate drought.
Forecast confidence for the Upper Midwest and northern Michigan is moderate.

Across western New York, northern Ohio, extreme northwest Pennsylvania and eastern Michigan, well-below normal streamflows have developed despite the area receiving some precipitation recently. CPC's 6-10 day, 8-14day and monthly precipitation forecasts favor greater odds for below-median precipitation here. Also, seasonal soil moisture and runoff forecasts call for below-normal runoff and soil moisture levels to continue. Though precipitation is expected to affect some of this region in the short-term, there is enough consensus amongst the tools for dry conditions in this region. Thus, drought development is now indicated for this region.
Forecast confidence for eastern Michigan, northern Ohio and western New York is moderate.

Abnormal dryness has recently developed across northern Kentucky into southern Ohio, with parts of north-central Kentucky placed into moderate drought. Below-normal soil moisture and streamflow conditions combined with lower precipitation totals than surrounding areas characterize conditions in this region. Though some rainfall is anticipated in the short-term, CPC's 6-10 day, monthly and seasonal precipitation forecasts indicate near to below normal precipitation for this region. A few seasonal soil moisture tools also suggest dry soils are likely to continue and also expand southward into the Tennessee Valley region. The official outlook indicates drought conditions to persist across central Kentucky, but shows some drought development from southern Ohio southward across central parts of Kentucky and Tennessee.
Forecast confidence for the lower Ohio and Tennessee Valley region is low.

In recent weeks, a series of disturbances has moved across the western and southwestern portion of the U.S.. However, most of the heaviest precipitation associated with these storm systems has not fallen across the Great Basin and the Four Corners region. This is reflected in two-week and 30-day percent of normal precipitation totals falling below 100 percent. NRCS SNOTEL percent of average snow water content values for northern California, southern Oregon and northern and central Nevada are at or slightly below normal. Snow water content values over northeast Arizona, however, are above normal due to a southern storm track during the winter months (consistent with El Niño winters) leading to a significant snowpack. CPC's 6-10 day and 8-14 day precipitation forecast for the Great Basin & the Four Corners region show a tilt in the odds for above-median precipitation. CPC's MJJ precipitation forecast calls for equal chances for above- or below-median precipitation for the Great Basin, with a slight tilt in the odds for above-median precipitation across a small portion of the northeast Arizona drought area. As the Great Basin/Four Corners region is entering its climatological dry season, even a small amount of precipitation would be considered above-median. Precipitation amounts from operational models through the week-2 period do not indicate substantial precipitation is likely from any storm(s). GFS-based Soil moisture forecasts through the week-2 period also show near to below normal soil moisture for the Great Basin/Four Corners region, though some increase in soil moisture for both areas is hinted at by a few of the seasonal soil moisture tools. Without evidence for significant precipitation in the short and medium-range model guidance and some conflicting evidence from soil moisture tools, drought persistence is forecast for the drought areas in northern California, Nevada, southern Oregon, and Arizona.
Forecast confidence for California, Nevada, southern Oregon and the Four Corners Region is moderate.

During the winter months, a typical El Niño winter atmospheric circulation pattern prevailed across the U.S. that featured an enhanced southern storm track. The enhanced southern track combined with above-normal temperatures across the northern Rockies to result in snowfall totals averaging well below normal across the eastern Pacific Northwest into the northern Rockies. Though snow has fallen recently due to current unsettled weather, NRCS SNOTEL snow water content percent of normal values across the region remain quite low, ranging between 50-90% of normal. USGS streamflow data across this region also indicates well below normal streamflows characterize this area. Soil moisture and runoff forecasts through the MJJ season are pessimistic on improvement across Idaho and western Montana. There is some indication, however, that soil moisture percentiles may increase across western Wyoming based on the week-1 and week-2 and seasonal soil moisture tools. Forecasts of precipitation for the interior Pacific Northwest and the northern and central Rockies show odds favoring above-median precipitation in the next few days and during the 6-10 day and 8-14 day period. CPC's official May precipitation probability forecast also favors slight odds toward above-median precipitation across Wyoming and Colorado. Given antecedent poor snow water content and streamflow conditions across Idaho and Montana and forecasts of below normal seasonal runoff and soil moisture forecasts in those areas, precipitation that is anticipated to fall in the short to medium range does not appear likely to yield any substantive improvement to drought conditions. Therefore, drought persistence is indicated for these states. Some improvement to drought conditions is indicated for parts of western Wyoming given more optimistic soil moisture forecasts here, with improvement to drought conditions for the drought area in northwest Colorado.
Forecast confidence for Montana and Idaho is moderate. Forecast confidence for Wyoming and Colorado is moderate.

Moderate to exceptional drought continues across the leeward side of the Hawaiian Islands, while trade showers have begun to increase in recent weeks on the windward side of the Hawaiian Islands. Forecasts for Hawaii call for below-median precipitation during the MJJ season, leading to continued drought persistence on the leeward side of the islands. Recent heavy rains on the northeastern side of Kauai and continued trade shower activity are likely to yield improvement on Kauai.
Forecast confidence for Hawaii is moderate.

Abnormal dryness currently exists in parts of central Alaska. Abnormal dryness has also spread slightly further south and west into parts of south-central Alaska based on the latest US Drought Monitor. Snow water content continues to remain well below normal across this region (at or below 50% of normal). Slight odds for below normal precipitation during the 6-10 day and 8-14 day period combined with warming temperatures later in the season lead to a continued forecast of drought development for central Alaska.
Forecast confidence for Alaska is low.